Kinds of mutations formed when a shuttle vector containing adducts of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9, 10-tetrahydrobenzo[a]pyrene replicates in human cells

DNA Repair and Mutagenesis of ADP-Ribosylated DNA by Pierisin

Pierisin is a DNA-targeting ADP-ribosyltransferase found in cabbage white butterfly (Pieris rapae). Pierisin transfers an ADP-ribosyl moiety to the 2-amino group of the guanine residue in DNA, yielding N2-(ADP-ribos-1-yl)-2'-deoxyguanosine (N2-ADPR-dG). Generally, such chemically modified DNA is recognized as DNA damage and elicits cellular responses, including DNA repair pathways. In Escherichia coli and human cells, it has been experimentally demonstrated that N2-ADPR-dG is a substrate of the nucleotide excision repair system. Although DNA repair machineries can remove most lesions, some unrepaired damages frequently lead to mutagenesis through DNA replication. Replication past the damaged DNA template is called translesion DNA synthesis (TLS). In vitro primer extension experiments have shown that eukaryotic DNA polymerase κ is involved in TLS across N2-ADPR-dG. In many cases, TLS is error-prone and thus a mutagenic process. Indeed, the induction of G:C to T:A and G:C to C:G mutations by N2-ADPR-dG in the hypoxanthine phosphoribosyltransferase gene mutation assay with Chinese hamster cells and supF shuttle vector plasmids assay using human fibroblasts has been reported. This review provides a detailed overview of DNA repair, TLS and mutagenesis of N2-ADPR-dG induced by cabbage butterfly pierisin-1.

Mutagenic Replication of N2-Deoxyguanosine Benzo[a]pyrene Adducts by Escherichia coli DNA Polymerase I and Sulfolobus solfataricus DNA Polymerase IV

Benzo[a]pyrene, a potent human carcinogen, is metabolized in vivo to a diol epoxide that reacts with the N²-position of guanine to produce N²-BP-dG adducts. These adducts are mutagenic causing G to T transversions. These adducts block replicative polymerases but can be bypassed by the Y-family translesion synthesis polymerases. The mechanisms by which mutagenic bypass occurs is not well-known. We have evaluated base pairing structures using atomic substitution of the dNTP with two stereoisomers, 2'-deoxy-N-[(7R,8S,9R,10S)-7,8,9,10-tetrahydro-7,8,9-trihydroxybenzo[a]pyren-10-yl]guanosine and 2'-deoxy-N-[(7S,8R,9S,10R)-7,8,9,10-tetrahydro-7,8,9-trihydroxybenzo[a]pyren-10-yl]guanosine. We have examined the kinetics of incorporation of 1-deaza-dATP, 7-deaza-dATP, 2'-deoxyinosine triphosphate, and 7-deaza-dGTP, analogues of dATP and dGTP in which single atoms are changed. Changes in rate will occur if that atom provided a critical interaction in the transition state of the reaction. We examined two polymerases, Escherichia coli DNA polymerase I (Kf) and Sulfolobus solfataricus DNA polymerase IV (Dpo4), as models of a high fidelity and TLS polymerase, respectively. We found that with Kf, substitution of the nitrogens on the Watson-Crick face of the dNTPs resulted in decreased rate of reactions. This result is consistent with a Hoogsteen base pair in which the template N²-BP-dG flipped from the anti to syn conformation. With Dpo4, while the substitution did not affect the rate of reaction, the amplitude of the reaction decreased with all substitutions. This result suggests that Dpo4 bypasses N²-BP-dG via Hoogsteen base pairs but that the flipped nucleotide can be either the dNTP or the template.

In vivo evidence that phenylalanine 171 acts as a molecular brake for translesion DNA synthesis across benzo[a]pyrene DNA adducts by human DNA polymerase κ

Humans possess multiple specialized DNA polymerases that continue DNA replication beyond a variety of DNA lesions. DNA polymerase kappa (Pol κ) bypasses benzo[a]pyrene diolepoxide-N(2)-deoxyguanine (BPDE-N(2)-dG) DNA adducts in an almost error-free manner. In the previous work, we changed the amino acids close to the adducts in the active site and examined the bypass efficiency. The substitution of alanine for phenylalanine 171 (F171A) enhanced by 18-fold in vitro, the efficiencies of dCMP incorporation opposite (-)- and (+)-trans-anti-BPDE-N(2)-dG. In the present study, we established human cell lines that express wild-type Pol κ (POLK+/-), F171A (POLK F171A/-) or lack expression of Pol κ (POLK-/-) to examine the in vivo significance. These cell lines were generated with Nalm-6, a human pre-B acute lymphoblastic leukemia cell line, which has high efficiency for gene targeting. Mutations were analyzed with shuttle vectors having (-)- or (+)-trans-anti-BPDE-N(2)-dG in the supF gene. The frequencies of mutations were in the order of POLK-/->POLK+/->POLK F171A/- both in (-)- and (+)-trans-anti-BPDE-N(2)-dG. These results suggest that F171 may function as a molecular brake for bypass across BPDE-N(2)-dG by Pol κ and raise the possibility that the cognate substrates for Pol κ are not BP adducts in DNA but may be lesions in DNA induced by endogenous mutagens.

Structural and thermodynamic insight into Escherichia coli UvrABC-mediated incision of cluster diacetylaminofluorene adducts on the NarI sequence

Cluster DNA damage refers to two or more lesions in a single turn of the DNA helix. Such clustering may occur with bulky DNA lesions, which may be responsible for their sequence-dependent repair and mutational outcomes. Here we prepared three 16-mer cluster duplexes in which two fluoroacetylaminofluorene adducts (dG-FAAF) are separated by zero, one, and two nucleotides in the Escherichia coli NarI mutational hot spot (5'-CTCTCG1G2CG3CCATCAC-3'): 5'-CG1*G2*CG3CC-3', 5'-CG1G2*CG3*CC-3', and 5'-CG1*G2CG3*CC-3' (G* = dG-FAAF), respectively. We conducted spectroscopic, thermodynamic, and molecular dynamics studies of these di-FAAF duplexes, and the results were compared with those of the corresponding mono-FAAF adducts in the same NarI sequence [Jain, V., et al. (2012) Nucleic Acids Res. 40, 3939-3951]. Our nucleotide excision repair results showed the diadducts were more reparable than the corresponding monoadducts. Moreover, we observed dramatic flanking base sequence effects on their repair efficiency in the following order: NarI-G2G3 > NarI-G1G3 > NarI-G1G2. The nuclear magnetic resonance, circular dichroism, ultraviolet melting, and molecular dynamics simulation results revealed that in contrast to the monoadducts, diadducts produced a synergistic effect on duplex destabilization. In addition, dG-FAAF at G2G3 and G1G3 destacks the neighboring bases, with greater destabilization occurring with the former. Overall, the results indicate the importance of base stacking and related thermal and thermodynamic destabilization in the repair of bulky cluster arylamine DNA adducts.

Characterization of TCHQ-induced genotoxicity and mutagenesis using the pSP189 shuttle vector in mammalian cells

Tetrachlorohydroquinone (TCHQ) is a major toxic metabolite of the widely used wood preservative, pentachlorophenol (PCP), and it has also been implicated in PCP genotoxicity. However, the underlying mechanisms of genotoxicity and mutagenesis induced by TCHQ remain unclear. In this study, we examined the genotoxicity of TCHQ by using comet assays to detect DNA breakage and formation of TCHQ-DNA adducts. Then, we further verified the levels of mutagenesis by using the pSP189 shuttle vector in A549 human lung carcinoma cells. We demonstrated that TCHQ causes significant genotoxicity by inducing DNA breakage and forming DNA adducts. Additionally, DNA sequence analysis of the TCHQ-induced mutations revealed that 85.36% were single base substitutions, 9.76% were single base insertions, and 4.88% were large fragment deletions. More than 80% of the base substitutions occurred at G:C base pairs, and the mutations were G:C to C:G, G:C to T:A or G:C to A:T transversions and transitions. The most common types of mutations in A549 cells were G:C to A:T (37.14%) and A:T to C:G transitions (14.29%) and G:C to C:G (34.29%) and G:C to T:A (11.43%) transversions. We identified hotspots at nucleotides 129, 141, and 155 in the supF gene of plasmid pSP189. These mutation hotspots accounted for 63% of all single base substitutions. We conclude that TCHQ induces sequence-specific DNA mutations at high frequencies. Therefore, the safety of using this product would be carefully examined.

Mutagenicity of tamoxifen DNA adducts in human endometrial cells and in silico prediction of p53 mutation hotspots

Tamoxifen elevates the risk of endometrial tumours in women and alpha-(N(2)-deoxyguanosinyl)-tamoxifen adducts are reportedly present in endometrial tissue of patients undergoing therapy. Given the widespread use of tamoxifen there is considerable interest in elucidating the mechanisms underlying treatment-associated cancer. Using a combined experimental and multivariate statistical approach we have examined the mutagenicity and potential consequences of adduct formation by reactive intermediates in target uterine cells. pSP189 plasmid containing the supF gene was incubated with alpha-acetoxytamoxifen or 4-hydroxytamoxifen quinone methide (4-OHtamQM) to generate dG-N(2)-tamoxifen and dG-N(2)-4-hydroxytamoxifen, respectively. Plasmids were replicated in Ishikawa cells then screened in Escherichia coli. Treatment with both alpha-acetoxytamoxifen and 4-OHtamQM caused a dose-related increase in adduct levels, resulting in a damage-dependent increase in mutation frequency for alpha-acetoxytamoxifen; 4-OHtamQM had no apparent effect. Only alpha-acetoxytamoxifen generated statistically different supF mutation spectra relative to the spontaneous pattern, with most mutations being GC-->TA transversions. Application of the LwPy53 algorithm to the alpha-acetoxytamoxifen spectrum predicted strong GC-->TA hotspots at codons 244 and 273. These signature alterations do not correlate with current reports of the mutations observed in endometrial carcinomas from treated women, suggesting that dG-N(2)-tam adduct formation in the p53 gene is not a prerequisite for endometrial cancer initiation in women.

Homology modeling of four Y-family, lesion-bypass DNA polymerases: the case that E. coli Pol IV and human Pol kappa are orthologs, and E. coli Pol V and human Pol eta are orthologs

Y-family DNA polymerases (DNAPs) are a superfamily of evolutionarily related proteins that exist in cells to bypass DNA damage caused by both radiation and chemicals. Cells have multiple Y-family DNAPs, presumably to conduct translesion synthesis (TLS) on DNA lesions of varying structure and conformation. The potent, ubiquitous environmental mutagen/carcinogen benzo[a]pyrene (B[a]P) induces all classes of mutations with G-->T base substitutions predominating. We recently showed that a G-->T mutagenesis pathway for the major adduct of B[a]P ([+ta]-B[a]P-N2-dG) in Escherichia coli depends on Y-family member DNAP V. Since no X-ray crystal study for DNAP V has been reported, no structure is available to help in understanding the structural basis for dATP insertion associated with G-->T mutations from [+ta]-B[a]P-N2-dG. Herein, we do homology modeling to construct a model for UmuC, which is the polymerase subunit of DNAP V. The sequences of eight Y-family DNAPs were aligned based on the positioning of conserved amino acids and an analysis of conserved predicted secondary structure, as well as insights gained from published X-ray structures of five Y-family members. Starting coordinates for UmuC were generated from the backbone coordinates for the Y-family polymerase Dpo4 for reasons discussed, and were refined using molecular dynamics with CHARMM 27. A survey of the literature revealed that E. coli DNAP V and human DNAP eta show a similar pattern of dNTP insertion opposite a variety of DNA lesions. Furthermore, E. coli DNAP IV and human DNAP kappa show a similar dNTP insertional pattern with these same DNA lesions, although the insertional pattern for DNAP IV/kappa differs from the pattern for DNAPs V/eta. These comparisons prompted us to construct and refine models for E. coli DNAP IV and human DNAPs eta and kappa as well. The dNTP/template binding pocket of all four DNAPs was inspected, focusing on the array of seven amino acids that contact the base of the incoming dNTP, as well as the template base. DNAPs V and eta show similarities in this array, and DNAPs IV and kappa also show similarities, although the arrays are different for the two pairs of DNAPs. Thus, there is a correlation between structural similarities and insertional similarities for the pairs DNAPs V/eta and DNAPs IV/kappa. Although the significance of this correlation remains to be elucidated, these observations point the way for future experimental studies.

Mutagenesis studies with four stereoisomeric N2-dG benzo[a]pyrene adducts in the identical 5′-CGC sequence used in NMR studies: G→T mutations dominate in each case

Benzo[a]pyrene (B[a]P) is a polycyclic aromatic hydrocarbon (PAH) and a potent mutagen/carcinogen found ubiquitously in the environment. B[a]P is primarily metabolized to diol epoxides, which react principally at N2-dG in DNA. B[a]P-N2-dG adducts have been shown to induce a variety of mutations, notably G-->T, G-->A, G-->C and -1 frameshifts. Four stereoisomers of B[a]P-N2-dG (designated: [+ta]-;, [+ca]-, [-ta] and [-ca]) were studied by NMR in duplex 11mers in a 5'-CGC sequence context, and each adopted a different adduct conformation (Geacintov, et al. (1997) Chem. Res. Toxicol., 10, 111). Herein these four identical B[a]P-containing 11mers are built into duplex plasmid genomes and mutagenesis studied in Escherichia coli following SOS-induction. In nucleotide excision repair (NER) proficient E.coli, no adduct-derived mutants are detected. In NER deficient E.coli, G-->T mutations dominate for all four stereoisomers [+ta]-, [+ca]-, [-ta] and [-ca]-B[a]P-N(2)-dG, and mutation frequency is similar. Thus, the mutagenic pattern for these four B[a]P-N2-dG stereoisomers is the same, in spite of the fact that they adopt dramatically different conformations in ds-oligonucleotides as determined by NMR. These findings suggest that adduct conformation must be fluid enough in the 5'-CGC sequence that the duplex DNA conformation can interconvert to mutagenic and non-mutagenic conformations during lesion-bypass. A comparison of all published studies with these four B[a]P-N2-dG stereoisomers in E.coli reveals that B[a]P-N2-dG adduct stereochemistry tends to have a lesser impact on mutagenic pattern (e.g. G-->T versus G-->A mutations) than does DNA sequence context, which is discussed.

Mutagenesis studies of the major benzo[a]pyrene N2-dG adduct in a 5'-TG versus a 5'-UG sequence: removal of the methyl group causes a modest decrease in the [G->T/G->A] mutational ratio

The potent mutagen/carcinogen benzo[a]pyrene (B[a]P) is metabolically activated to (+)-anti-B[a]PDE, which induces a full spectrum of mutations primarily at the G:C base pairs (e.g. GC-->TA, GC-->AT, etc.). Each of these mutations can be induced by its major adduct [+ta]-B[a]P-N(2)-dG, where DNA sequence context appears to influence both the quantitative and qualitative pattern of mutagenesis. We noted previously that 5'-TG sequences tend to have a higher fraction of G-->T mutations for both [+ta]-B[a]P-N(2)-dG and (+)-anti-B[a]PDE in comparison with 5'-CG, 5'-GG or 5'-AG sequences. To investigate a possible structural element for this trend, the role (if any) of the methyl group on the 5'-T is considered. Using adduct site-specific means, the [G-->T/G-->A] mutational ratio for [+ta]-B[a]P-N(2)-dG is determined to be approximately 1.08 in a 5'-TGT sequence, and approximately 0.60 in a 5'-UGT sequence. (G-->C mutations are minor.) Although this modest approximately 1.8-fold decrease in [G-->T/G-->A] ratio is statistically significant (P = 0.03), it suggests that the methyl group on the 5'-T is not the main reason why a 5'-T tends to enhance G-->T mutations. This study was prompted by an adduct conformational hypothesis, which predicted that the removal of the methyl group in a 5'-TG sequence would lower the fraction of G-->T mutations; however, the approximately 1.8-fold decrease is too small to do additional experiments to assess whether this conformational hypothesis, or other hypotheses, are the true cause of the decrease, which is discussed in this paper.

Molecular modeling of the major benzo[a]pyrene N2-dG adduct in cases where mutagenesis results are known in double stranded DNA

The potent mutagen/carcinogen benzo[a]pyrene (B[a]P) is metabolically activated to (+)-anti-B[a]PDE, which induces a full spectrum of mutations (e.g. GC-->TA, GC-->AT, etc.). One hypothesis for this complexity is that different mutations are induced by different conformations of its major adduct [+ta]-B[a]P-N2-dG when bypassed during DNA replication (probably by different DNA polymerases). Previous molecular modeling studies suggested that B[a]P-N2-dG adducts can in principle adopt at least 16 potential conformational classes in ds-DNA. Herein we report on molecular modeling studies with the eight conformations most likely to be relevant to base substitution mutagenesis in 10 cases where mutagenesis has been studied in ds-DNA plasmids in E. coli with B[a]P-N2-dG adducts of differing stereoisomers and DNA sequence contexts, as well as in five cases where the conformation is known by NMR. Of the approximately 11,000 structures generated in this study, the computed lowest energy structures are reported for 120 cases (i.e. eight conformations and 15 examples), and their conformations compared. Of the eight conformations, four are virtually always computed to be high in energy. The remaining four lower energy conformations include two with the BP moiety in the minor groove (designated: BPmi5 and BPmi3), and two base-displaced conformations, one with the dG moiety in the major groove (designated: Gma5) and one with the dG in the minor groove (designated: Gmi3). Interestingly, these four are the only conformations that have been observed for B[a]P-N2-dG adducts in NMR studies. Independent of sequence contexts and adduct stereochemistry, BPmi5 structures tend to look reasonably similar, as do BPmi3 structures, while the base-displaced structures Gma5 and BPmi3 tend to show greater variability in structure. A correlation was sought between modeling and mutagenesis results in the case of the low energy conformations BPmi5, BPmi3, Gma5 and Gma3. Plots of log[(G-->T)/(G-->A)] versus energy[(conformation X)-(conformation Y)] were constructed for all six pairwise combinations of these four conformations, and the only plot giving a straight line involved Gma5 and Gmi3. While this finding is striking, its significance is unclear (as discussed).

Malondialdehyde, a product of lipid peroxidation, is mutagenic in human cells

Malondialdehyde (MDA) is an endogenous genotoxic product of enzymatic and oxygen radical-induced lipid peroxidation whose adducts are known to exist in DNA isolated from healthy human beings. To evaluate the mutagenic potential of MDA in human cells, we reacted MDA with pSP189 shuttle vector DNA and then transfected them into human fibroblasts for replication. MDA induced up to a 15-fold increase in mutation frequency in the supF reporter gene compared with untreated DNA. Sequence analysis revealed that the majority of MDA-induced mutations occurred at GC base pairs. The most frequent mutations were large insertions and deletions, but base pair substitutions were also detected. MDA-induced mutations were completely abolished when the adducted shuttle vector was replicated in cells lacking nucleotide excision repair. MDA induction of large deletions and the apparent requirement for nucleotide excision repair suggested the possible involvement of a DNA interstrand cross-link as a premutagenic lesion. Indeed, MDA formed interstrand cross-links in duplex plasmids and oligonucleotides. Substrates containing the sequence 5'-d(CG) were preferentially cross-linked, consistent with the observation of base pair substitutions in 5'-d(CG) sites in the MDA-induced mutation spectrum. These experiments provide biological and biochemical evidence for the existence of MDA-induced DNA interstrand cross-links that could result from endogenous oxidative stress and likely have potent biological effects.

A critical evaluation of DNA adducts as biological markers for human exposure to polycyclic aromatic compounds

The causative role of polycyclic aromatic hydrocarbons (PAH) in human carcinogenesis is undisputed. Measurements of PAH-DNA adduct levels in easily accessible white blood cells therefore represent useful early endpoints in exposure intervention or chemoprevention studies. The successful applicability of DNA adducts as early endpoints depends on several criteria: i. adduct levels in easily accessible surrogate tissues should reflect adduct levels in target-tissues, ii. toxicokinetics and the temporal relevance should be properly defined. iii. sources of interand intra-individual variability must be known and controllable, and finally iv. adduct analyses must have advantages as compared to other markers of PAHexposure. In general, higher DNA adduct levels or a higher proportion of subjects with detectable DNA adduct levels were found in exposed individuals as compared with nonexposed subjects, but saturation may occur at high exposures. Furthermore, DNA adduct levels varied according to changes in exposure, for example smoking cessation resulted in lower DNA adduct levels and adduct levels paralleled seasonal variations of air-pollution. Intraindividual variation during continuous exposure was low over a short period of time (weeks), but varied significantly when longer time periods (months) were investigated. Inter-individual variation is currently only partly explained by genetic polymorphisms in genes involved in PAH-metabolism and deserves further investigation. DNA adduct measurements may have three advantages over traditional exposure assessment: i. they can smooth the extreme variability in exposure which is typical for environmental toxicants and may integrate exposure over a longer period of time. Therefore, DNA adduct assessment may reduce the monitoring effort. ii. biological monitoring of DNA adducts accounts for all exposure routes. iii. DNA adducts may account for inter-individual differences in uptake, elimination, distribution, metabolism and repair amongst exposed individuals. In conclusion, there is now a sufficiently large scientific basis to justify the application of DNA adduct measurements as biomarkers in exposure assessment and intervention studies. Their use in risk-assessment, however, requires further investigation.

Translesion replication of benzo[a]pyrene and benzo[c]phenanthrene diol epoxide adducts of deoxyadenosine and deoxyguanosine by human DNA polymerase iota

Human DNA polymerase iota (poliota) is a Y-family polymerase whose cellular function is presently unknown. Here, we report on the ability of poliota to bypass various stereoisomers of benzo[a]pyrene (BaP) diol epoxide (DE) and benzo[c]phenanthrene (BcPh) DE adducts at deoxyadenosine (dA) or deoxyguanosine (dG) bases in four different template sequence contexts in vitro. We find that the BaP DE dG adducts pose a strong block to poliota-dependent replication and result in a high frequency of base misincorporations. In contrast, misincorporations opposite BaP DE and BcPh DE dA adducts generally occurred with a frequency ranging between 2 x 10(-3) and 6 x 10(-4). Although dTMP was inserted efficiently opposite all dA adducts, further extension was relatively poor, with one exception (a cis opened adduct derived from BcPh DE) where up to 58% extension past the lesion was observed. Interestingly, another human Y-family polymerase, polkappa, was able to extend dTMP inserted opposite a BaP DE dA adduct. We suggest that poliota might therefore participate in the error-free bypass of DE-adducted dA in vivo by predominantly incorporating dTMP opposite the damaged base. In many cases, elongation would, however, require the participation of another polymerase more specialized in extension, such as polkappa.

Mutational spectra of benzo[a]pyrene and MeIQx in rpsL transgenic zebrafish embryos

To evaluate the rpsL transgenic zebrafish (Brachydanio rerio) mutation assay, we treated the embryos with benzo[a]pyrene (B[a]P) (10 microg/ml) or 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) (300 microg/ml) for 16h and determined the mutation spectra. These treatments were previously reported to induce mutant frequencies that were 4.3 and 2.4 times the control value, respectively. In the B[a]P-treated group, half of the mutations were single base substitutions, 74% of which occurred at G:C base pairs. Among G:C base pair substitutions, G:C to T:A and G: C to C:G transversions were predominant, suggesting that B[a]P induced mutations in zebrafish embryos by mechanisms previously described in mammalian tissues. In the MeIQx-treated group, about 60% of the mutations were deletions. Some specific mutations were found, but the compound primarily amplified the background mutation level; improvement in the conditions of treatment may be required for elucidating MeIQx-mutagenesis in this system. This study showed that transgenic zebrafish may be a useful tool for detecting mutagens in aquatic environments and for elucidating mutagenic mechanisms.

Factors that influence the mutagenic patterns of DNA adducts from chemical carcinogens

Carcinogens are generally mutagens, which is understandable given that tumor cells grow uncontrollably because they have mutations in critical genes involved in growth control. Carcinogens often induce a complex pattern of mutations (e.g., GC-->TA, GC-->AT, etc.). These mutations are thought to be initiated when a DNA polymerase encounters a carcinogen-DNA adduct during replication. In principle, mutational complexity could be due to either a collection of different adducts each inducing a single kind of mutation (Hypothesis 1a), or a single adduct inducing different kinds of mutations (Hypothesis 1b). Examples of each are discussed. Regarding Hypothesis 1b, structural factors (e.g., DNA sequence context) and biological factors (e.g., differing DNA polymerases) that can affect the pattern of adduct mutagenesis are discussed. This raises the question: how do structural and biological factors influence the pattern of adduct mutagenesis. For structural factors, three possibilities are considered: (Hypothesis 2a) a single conformation of an adduct giving rise to multiple mutations -- dNTP insertion by DNA polymerase being influenced by (e.g.) the surrounding DNA sequence context; (Hypothesis 2b) a variation on this ("dislocation mutagenesis"); or (Hypothesis 2c) a single adduct adopting multiple conformations, each capable of giving a different pattern of mutations. Hypotheses 2a, 2b and 2c can each in principle rationalize many mutational results, including how the pattern of adduct mutagenesis might be influenced by factors, such as DNA sequence context. Five lines of evidence are discussed suggesting that Hypothesis 2c can be correct for base substitution mutagenesis. For example, previous work from our laboratory was interpreted to indicate that [+ta]-B[a]P-N(2)-dG in a 5'-CGG sequence context (G115) could be trapped in a conformation giving predominantly G-->T mutations, but heating caused the adduct to equilibrate to its thermodynamic mixture of conformations, leading to a decrease in the fraction of G-->T mutations. New work is described suggesting that [+ta]-B[a]P-N(2)-dG at G115 can also be trapped predominantly in the G-->A mutational conformation, from which equilibration can also occur, leading to an increase in the fraction of G-->T mutations. Evidence is also presented that the fraction of G-->T mutations is higher when [+ta]-B[a]P-N(2)-dG at G115 is in ss-DNA ( approximately 89%) vs. ds-DNA ( approximately 66%), a finding that can be rationalized if the mixture of adduct conformations is different in ss- and ds-DNA. In summary, the factors affecting adduct mutagenesis are reviewed and five lines of evidence that support one hypothesis (2c: adduct conformational complexity can cause adduct mutational complexity) are discussed.

Mutational spectra for polycyclic aromatic hydrocarbons in the supF target gene

An SV40-based shuttle vector system was used to identify the types of mutational changes and the sites of mutation within the supF DNA sequence generated by the four stereoisomers of benzo[c]phenanthrene 3,4-dihydrodiol 1,2-epoxide (B[c]PhDE), by racemic mixtures of bay or fjord region dihydrodiol epoxides (DE) of 5-methylchrysene, of 5, 6-dimethylchrysene, of benzo[g]chrysene and of 7-methylbenz[a]anthracene and by two direct acting polycyclic aromatic hydrocarbon carcinogens, 7-bromomethylbenz[a]anthracene (7-BrMeBA) and 7-bromomethyl-12-methylbenz[a]anthracene (7-BrMe-12-MeBA). The results of these studies demonstrated that the predominant type of mutation induced by these compounds is the base substitution. The chemical preference for reaction at deoxyadenosine (dAdo) or deoxyguanosine (dGuo) residues in DNA, which is in general correlated with the spatial structure (planar or non-planar) of the reactive polycyclic aromatic hydrocarbon, is reflected in the preference for mutation at A&z.ccirf;T or G&z.ccirf;C pairs. In addition, if the ability to react with DNA in vivo is taken into account, the relative mutagenic potencies of the B[c]PhDE stereoisomers are consistent with the higher tumorigenic activity associated with non-planar polycyclic aromatic hydrocarbons and their extensive reaction with dAdo residues in DNA. Comparison of the types of mutations generated by polycyclic aromatic hydrocarbons and other bulky carcinogens in this shuttle vector system suggests that all bulky lesions may be processed by a similar mechanism related to that involved in replication past apurinic sites. However, inspection of the distribution of mutations over the target gene induced by the different compounds demonstrated that individual polycyclic aromatic hydrocarbons induce unique patterns of mutational hotspots within the target gene. A polymerase arrest assay was used to determine the sequence specificity of the interaction of reactive polycyclic aromatic hydrocarbons with the shuttle vector DNA. The results of these assays revealed a divergence between mutational hotspots and polymerase arrest sites for all compounds investigated, i.e., sites of mutational hotspots do not correspond to sites where high levels of adduct formation occur, and suggested that some association between specific adducts and sequence context may be required to constitute a premutagenic lesion. A site-specific mutagenesis system employing a single-stranded vector (M13mp7L2) was used to investigate the mutational events a single benzo[a]pyrene or benzo[c]phenanthrene dihydrodiol epoxide-DNA adduct elicits within specific sequence contexts. These studies showed that sequence context can cause striking differences in mutagenic frequencies for given adducts. In addition, these sequence context effects do not originate only from nucleotides immediately adjacent to the adduct, but are also modulated by more distal nucleotides. The implications of these results for mechanisms of polycyclic aromatic hydrocarbon-induced mutagenesis and carcinogenesis are discussed.

Toward an understanding of the role of DNA adduct conformation in defining mutagenic mechanism based on studies of the major adduct (formed at N(2)-dG) of the potent environmental carcinogen, benzo[a]pyrene

The process of carcinogenesis is initiated by mutagenesis, which often involves replication past damaged DNA. One question - what exactly is a DNA polymerase seeing when it incorrectly copies a damaged DNA base (e.g., inserting dATP opposite a dG adduct)? - has not been answered in any case. Herein, we reflect on this question, principally by considering the mutagenicity of one activated form of benzo[a]pyrene, (+)-anti-B[a]PDE, and its major adduct [+ta]-B[a]P-N(2)-dG. In previous work, [+ta]-B[a]P-N(2)-dG was shown to be capable of inducing>95% G-->T mutations in one sequence context (5'-TGC), and approximately 95% G-->A mutations in another (5'-AGA). This raises the question - how can a single chemical entity induce different mutations depending upon DNA sequence context? Our current working hypothesis is that adduct conformational complexity causes adduct mutational complexity, where DNA sequence context can affect the former, thereby influencing the latter. Evidence supporting this hypothesis was discussed recently (Seo et al., Mutation Res. [in press]). Assuming this hypothesis is correct (at least in some cases), one goal is to consider what these mutagenic conformations might be. Based on molecular modeling studies, 16 possible conformations for [+ta]-B[a]P-N(2)-dG are proposed. A correlation between molecular modeling and mutagenesis work suggests a hypothesis (Hypothesis 3): a base displaced conformation with the dG moiety of the adduct in the major vs. minor groove gives G-->T vs. G-->A mutations, respectively. (Hypothesis 4, which is a generalized version of Hypothesis 3, is also proposed, and can potentially rationalize aspects of both [+ta]-B[a]P-N(2)-dG and AP-site mutagenesis, as well as the so-called "A-rule".) Finally, there is a discussion of how conformational complexity might explain some unusual mutagenesis results that suggest [+ta]-B[a]P-N(2)-dG can become trapped in different conformations, and why we think it makes sense to interpret adduct mutagenesis results by modeling ds-DNA (at least in some cases), even though the mutagenic event must occur at a ss/ds-DNA junction in the presence of a DNA polymerase.

Targeting of lung cancer mutational hotspots by polycyclic aromatic hydrocarbons

BACKGROUND

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in combustion products of organic matter, including cigarette smoke. Metabolically activated diol epoxides of these compounds, including benzo[a]pyrene diol epoxide (B[a]PDE), have been suggested as causative agents in the development of lung cancer. We previously mapped the distribution of B[a]PDE adducts within the p53 tumor suppressor gene (also known as TP53), which is mutated in 60% of human lung cancers, and found that B[a]PDE adducts preferentially form at lung cancer mutational hotspots (codons 154, 157, 158, 245, 248, and 273). Other PAHs may be important in lung cancer as well.

METHODS

Here we have mapped the distribution of adducts induced by diol epoxides of additional PAHs: chrysene (CDE), 5-methylchrysene (5-MCDE), 6-methylchrysene (6-MCDE), benzo[c]phenanthrene (B[c]PDE), and benzo[g]chrysene (B[g]CDE) within exons 5, 7, and 8 of the p53 gene in human bronchial epithelial cells.

RESULTS

CDE exposure produced only low levels of adducts. Exposure of cells to the other activated PAHs resulted in DNA damage patterns similar to those previously observed with B[a]PDE but with some distinct differences. 5-MCDE, 6-MCDE, B[g]CDE, and B[c]PDE efficiently induced adducts at guanines within codons 154, 156, 157, 158, and 159 of exon 5, codons 237, 245 and 248 of exon 7, and codon 273 of exon 8, but the relative levels of adducts at each site varied for each compound. B[g]CDE, B[c]PDE, and 5-MCDE induced damage at codon 158 more selectively than 6-MCDE or B[a]PDE. The sites most strongly involved in PAH adduct formation were also the sites of highest mutation frequency (codons 157, 158, 245, 248, and 273).

CONCLUSION

The data suggest that PAHs contribute to the mutational spectrum in human lung cancer.

LacI mutation spectra following benzo[a]pyrene treatment of Big Blue mice

The mutation spectrum of the lacI gene from the liver of C57Bl6 Big Blue transgenic mice treated with benzo[a]pyrene (B[a]P) has been compared with the spectrum of spontaneous mutations observed in the liver of untreated Big Blue mice. Mice were treated with B[a]P for 3 days followed by a partial hepatectomy one day after the last injection. Liver tissue was removed for analysis at hepatectomy and, again, 3 days later at the time of sacrifice. Earlier, we reported that the lacI mutant frequency in these B[a]P-treated mice was elevated in the liver both at the time of hepatectomy and at sacrifice; however, a statistically significant increase in the mutant frequency was observed only at sacrifice. In this study, the DNA sequence spectra of lacI mutations observed in the liver of B[a]P-treated Big Blue mice at hepatectomy and at time of sacrifice were compared with each other and with the spectrum of spontaneous liver mutations. No differences were observed between the two B[a]P-treatment spectra. However, mutation frequencies of both GC-->TA and GC-->CG at the time of hepatectomy and at sacrifice were significantly elevated compared with the spontaneous frequency of these same transversions. Also, the frequency of AT-->TA transversions was significantly higher than the spontaneous frequency at the time of hepatectomy but not at sacrifice. The frequency of all other classes of mutations scored was not significantly different from the frequency of these same events in the spontaneous spectra. These data support the view that B[a]P treatment results in the induction of GC-->TA and GC-->CG transversions within 1 day of the last injection and they provide insights regarding the relative roles of benzo[a]pyrene-7,8-diol-9, 10-epoxide and radical cations of B[a]P in B[a]P-induced mutagenesis in vivo. Finally, these data provide evidence for B[a]P-induced mutagenesis under conditions where no statistical increase in mutant frequency could be shown.

Increase in mutation frequency in lung of Big Blue rat by exposure to diesel exhaust

Exposure to diesel exhaust (DE) is known to cause lung tumors in rats. To clarify the mutagenicity of DE, we estimated mutant frequency (MF) and determined the mutation spectra in rat lung after exposure to DE using lambda/lacI transgenic rats (Big Blue system). Male Big Blue rats (6 weeks old) were exposed for 4 weeks to 1 or 6 mg/m(3) DE, which contains suspended particulate matter. Control rats were maintained in filtered clean air. After exposure to 6 mg/m(3) DE, MF in lung was 4.8-fold higher than in control rats (P < 0.01), but no increase in MF was observed in rats exposed to 1 mg/m(3) DE. Sixty-nine mutants were identified after exposure to 6 mg/m(3) DE. The major mutations were A:T-->G:C (18 mutations) and G:C-->A:T (19 mutations) transitions. Remarkably, G-->T transversion of the lacI gene at site 221 was a hot-spot induced by exposure to DE, and there were complex mutations in which multiple mutations occurred in a single mutant, especially in the rats exposed to 6 mg/m(3) DE. DNA adducts formed by DE were analyzed using a (32)P-post-label TLC method and the amount of 8-hydroxydeoxyguanosine (8-OHdG) was measured using HPLC. Relative adduct level and amount of 8-OHdG were significantly increased in the rats exposed to 6 mg/m(3) DE compared with the controls (3.0- and 2.2-fold, respectively; P < 0.01). The level of cytochrome P450 1A1 mRNA was shown by northern blot analysis to be significantly increased in the lungs of rats exposed to 6 mg/m(3) DE (5.5-fold; P < 0.01). These results indicate that DE causes lesions in genomic DNA and acts as a mutagen in rat lung.

Fluorescence line-narrowing detection in chromatography and electrophoresis

A review of the basic aspects of fluorescence line-narrowing spectroscopy (FLNS) and its coupling with thin-layer chromatography (TLC) and polyacrylamide gel electrophoresis (PAGE) for off-line high-resolution low temperature spectral characterization is discussed. This is followed by a description of the on-line interfacing of capillary electrophoresis (CE) and capillary electrochromatography (CEC) with FLN detection. CE/ CEC-FLNS instrumentation and its applications for spectral identification of closely related analytes are also presented. Future prospects of micro and capillary high performance liquid chromatography (HPLC) with on-line high-resolution low temperature spectroscopic identification are considered.

Multiple mutations in a shuttle vector modified by ultraviolet irradiation, (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, and aflatoxin B(1) have different properties than single mutations and may be generated during translesion synthesis

Shuttle vector-based systems are extensively employed to study the mutational properties of various mutagens in mammalian cells. Such vectors are designed for the detection of point mutations, that is small deletions and single base and tandem substitutions. However, mutant target genes carrying two or more point mutations, referred to as multiple mutations, can also be found in various proportions depending on the mutagen and the cells used. To evaluate the frequency and characteristics of multiple mutations, we used a system where the plasmid, pYZ289, was treated by ultraviolet irradiation, aflatoxin B(1) or (+/-)-7 beta,8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene before transfection into mouse fibroblast cells. The kinds of mutations and the mutational spectra were different for single and multiple mutations. In addition, in at least 75% of the cases, mutations of multiples appeared to arise in the same strand. Furthermore, mutational spectra for multiple mutations were different for 5' and 3' members of multiple sets. These observations suggest that multiple mutations arise via a different mechanism than single mutations. Moreover, these findings suggest that multiples arise during translesion DNA synthesis and involve an error-prone polymerase able to introduce a base opposite misinstructive or noninstructional DNA lesions and subject to subsequent misincorporation errors.

The p53 tumor suppressor protein reduces point mutation frequency of a shuttle vector modified by the chemical mutagens (+/-)7, 8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, aflatoxin B1 and meta-chloroperoxybenzoic acid

p53 has been postulated to be the guardian of the genome. However, results supporting the prediction that point mutation frequencies are elevated in p53-deficient cells either have not been forthcoming or have been equivocal. To analyse the effect of p53 on point mutation frequency, we used the supF gene of the pYZ289 shuttle vector as a mutagenic target. pYZ289 was treated in vitro by ultraviolet irradiation, aflatoxin B1, (+/-)7,8-dihydroxy-9, 10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene and meta-chloroperoxybenzoic acid and then transfected into p53-deficient cells with or without a p53 expression vector. p53 reduced the mutant frequency up to fivefold when pYZ289 was treated with aflatoxin B1, (+/-)7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene or meta-chloroperoxybenzoic acid but not when it was ultraviolet-irradiated. The p53-dependent mutation frequency reduction was higher at a higher level of premutational lesions for aflatoxin B1 and (+/-)7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene and at a lower level of lesions for meta-chloroperoxybenzoic acid. This suggests that the chemical mutagens produce, in a dose-dependent fashion, two kinds of DNA damage, one subject to p53-dependent mutation frequency reduction and the other not. These results indicate that p53 can reduce the point mutation frequency in a shuttle vector treated by chemical mutagens and suggest that p53 can act as guardian of the genome for at least some kinds of point mutations.

Molecular analysis of mutations induced by a benzene metabolite, p-benzoquinone, in mouse cells using a novel shuttle vector plasmid

Human population has been continually exposed to benzene which is present in our environment as an essential component of petroleum. p-Benzoquinone (p-BQ) is one of the benzene metabolites and is thought to be an ultimate toxic or carcinogenic substance. For molecular analysis of carcinogen-induced mutations in mouse cells, we constructed a new shuttle vector plasmid pNY200 that has supF gene as a target of the mutations and replicates in mouse and in Escherichia coli cells. In p-BQ-treated pNY200 propagated in mouse cells, base substitutions were induced predominantly at G:C sites, and the major mutation was G:C-->A:T transition. Many tandem base substitutions were also induced at CC:GG sequences. By a postlabeling analysis and a polymerase stop assay, we confirmed that p-BQ adducts formed in DNA and mutation sites roughly correspond to the sites where the adducts were formed. Comparing data of pNY200 in mouse cells with those of the similar shuttle vector plasmid pMY189 in human cells should be important for extrapolation of data from mouse to human, because carcinogenicity of chemicals is tested in mice.

Cell cycle control, checkpoint mechanisms, and genotoxic stress

The ability of cells to maintain genomic integrity is vital for cell survival and proliferation. Lack of fidelity in DNA replication and maintenance can result in deleterious mutations leading to cell death or, in multicellular organisms, cancer. The purpose of this review is to discuss the known signal transduction pathways that regulate cell cycle progression and the mechanisms cells employ to insure DNA stability in the face of genotoxic stress. In particular, we focus on mammalian cell cycle checkpoint functions, their role in maintaining DNA stability during the cell cycle following exposure to genotoxic agents, and the gene products that act in checkpoint function signal transduction cascades. Key transitions in the cell cycle are regulated by the activities of various protein kinase complexes composed of cyclin and cyclin-dependent kinase (Cdk) molecules. Surveillance control mechanisms that check to ensure proper completion of early events and cellular integrity before initiation of subsequent events in cell cycle progression are referred to as cell cycle checkpoints and can generate a transient delay that provides the cell more time to repair damage before progressing to the next phase of the cycle. A variety of cellular responses are elicited that function in checkpoint signaling to inhibit cyclin/Cdk activities. These responses include the p53-dependent and p53-independent induction of Cdk inhibitors and the p53-independent inhibitory phosphorylation of Cdk molecules themselves. Eliciting proper G1, S, and G2 checkpoint responses to double-strand DNA breaks requires the function of the Ataxia telangiectasia mutated gene product. Several human heritable cancer-prone syndromes known to alter DNA stability have been found to have defects in checkpoint surveillance pathways. Exposures to several common sources of genotoxic stress, including oxidative stress, ionizing radiation, UV radiation, and the genotoxic compound benzo[a]pyrene, elicit cell cycle checkpoint responses that show both similarities and differences in their molecular signaling.

Nickel subsulfide is similar to potassium dichromate in protecting normal human fibroblasts from the mutagenic effects of benzo[a]pyrene diolepoxide

The cellular response to multiple carcinogen treatment has not been extensively studied, even though the effect of individual carcinogens is, in many cases, well known. We have previously shown that potassium dichromate can protect normal human fibroblasts from the mutagenic effects of benzo[a]pyrene diolepoxide (BPDE), and that this effect may be via an oxidative stress mechanism [Tesfai et al. (1998) Mutat Res 416:159-168]. Here, we extend our previous work by showing that nickel subsulfide can produce the some effect. Normal human fibroblasts, preincubated with nickel subsulfide for 46 hr followed by a coincubation of nickel subsulfide and BPDE for 2 hr, showed a dramatic reduction in the mutant frequency of the hypoxanthine (guanine)phosphoribosyl-transferase (HPRT) gene when compared to cells treated only with BPDE. The preincubation period with nickel subsulfide was necessary to see the antagonistic effect, since it was not observed if the cells were simply incubated with both carcinogens for 2 hr. The extent of the antagonistic effect was nickel subsulfide dose-dependent and also appeared to be species-specific, since the effect was not observed when Chinese hamster fibroblasts were tested. Finally, the antagonistic effect of the nickel subsulfide was eliminated by vitamin E, suggesting that production of reactive oxygen species by the nickel may be required. This data, along with our previous work, suggest that the antagonistic effect we observe is not chromium-specific, and that it could be species-specific.

Chromium can reduce the mutagenic effects of benzo[a]pyrene diolepoxide in normal human fibroblasts via an oxidative stress mechanism

The interaction of multiple carcinogens on human cells has not been extensively examined. This study reports the results of experiments in which normal human fibroblasts were exposed to both benzo[a]pyrene diolepoxide (BPDE) and potassium dichromate. The effect of four different treatment protocols on the cloning ability of the cells and the mutant frequency of the HPRT gene was determined. The combined treatment of both carcinogens caused a slightly greater than additive decrease in the cloning ability of the cells when compared to cells treated with the individual carcinogens. The result was the same regardless of the treatment protocol used in the experiment. The results of the mutant frequency experiments, however, varied dramatically with the protocol employed. The mutant frequency in cells which were simultaneously treated with both carcinogens was dramatically reduced from the mutant frequency found when cells were treated with BPDE alone. This antagonistic effect was not present when cells were either pretreated with potassium dichromate prior to BPDE or incubated with potassium dichromate following BPDE treatment. The observed antagonistic effect was the result of oxidative stress produced by chromium since it was completely or nearly completely reversed by the addition of either vitamin E or catalase to the cultures.

Mutation spectra resulting from carcinogenic exposure: from model systems to cancer-related genes

The events leading to cancer are complex and interactive. Alteration of cancer genes, such as the tumor suppressor gene p53, plays a central role in this process. Analysis of the frequency, type and site of mutations in important cancer-related genes may provide clues to the identification of etiological factors and sources of exposure. In this chapter we have selected a few examples of environmental human carcinogens and have attempted to use the knowledge of their mechanisms of mutagenesis, as derived from in vitro cell systems, as a key to understanding the complexity of p53 mutation spectra in tumors arising at the putative target organ. The analysis will focus on environmental exposure to UV radiation. The examples of tobacco smoke, dietary aflatoxin and vinyl chloride will be also briefly discussed.

Molecular characterization of mutation and comparison of mutation profiles in the hprt gene of Chinese hamster ovary cells treated with benzo[a]pyrene trans-7,8-diol-anti-9,10-epoxide, 1-nitrobenzo[a]pyrene trans-7,8-diol-anti-9,10-epoxide, and 3-nitrobenzo[a]pyrene trans-7,8- diol-anti-9,10-epoxide

Both 1- and 3-nitrobenzo[a]pyrene (nitro-BaP) are environmental contaminants, potent mutagens in Salmonella, and moderate mutagens in Chinese hamster ovary (CHO) cells. The mutagenicity of their oxidized metabolites,trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-epoxy -7,8,9,10-tetrahydro-1-nitrobenzo[a]pyrene (1-nitro-BaP-DE) and trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydro-3-nitrobenzo[a]- pyrene (3-nitro-BaPDE), together with trans-7,8-dihydroxy-anti-9, 10-ep- oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BaP-DE), was determined in CHO-K1 cells, and the resulting mutations at the hprt locus were characterized by polymerase chain reaction (PCR) amplification of reverse-transcribed hprt mRNA, followed by DNA sequence analysis. The mutant frequencies, in mutants/10(6) clonable cells, at 30 and 100 ng/ml, were BaP-DE, 248 and 456; 1-nitro-BaP-DE, 68 and 260; 3-nitro-BaP-DE, 81 and 232, respectively. In general, the three diolepoxides exhibited similar mutational spectra: 1) 64% (23/36 sequenced mutants) of BaP-DE, 53% (19/36) of 1-nitro-BaP-DE, and 64% (23/36) of 3-nitro-BaP-DE mutants resulted from simple base pair substitution, with the predominant mutation being G-->T transversion; 2) 90%, 100%, and 100% of mutations at G:C had the mutated dG on the nontranscribed DNA strand; and 3) about one quarter of the mutants produced by each mutagen had one or more PCR products with partial or complete exon deletions. The mutagens induced few frameshifts or complex mutations. Among the differences in mutational specificity for the three diolepoxides, the proportion of substituted dGs with 3' purines was significant (P < 0.05) for BaP-DE (16/19, 84%) and 3-nitro-BaP-DE (17/20, 85%), but not significant for 1-nitro-BaP-DE-induced mutants (11/17, 65%, P > 0.05). Also, high proportions of BaP-DE and 3-nitro-BaP-DE base pair substitutions at G:C occurred in DNA sequence contexts of 5'-GG-3', 5'-GGA-3', and 5'-TGGA-3', while the proportions of 1-nitro-BaP-DE mutants in these contexts were often lower. The results indicate that nitro substitution at C1 or C3 of BaP-DE reduces mutational potency in CHO cells and appears to have only subtle effects upon the mutational pattern in the hprt gene.

Ultraviolet-B-light-induced mutagenesis of C-H-ras codons 11 and 12 in human skin fibroblasts

Mutations in the ras oncogene are detected with a high frequency in non-melanoma skin cancer. Approximately half of the squamous-cell carcinomas (SCC) and one third of the basal-cell carcinomas (BCC) carry mutations at the second position of Ha-ras codon 12 (GGC to GTC), whereas mutations in Ki-ras codon 12 occur less frequently. Since the mutations in the Ha-ras and Ki-ras oncogenes are located opposite potential pyrimidine dimer sites (C-C), it is likely that the mutations are induced by ultraviolet radiation present in sunlight. We studied the capacity of ultraviolet B (UVB) light to induce base-pair changes in Ha-ras codons 11 and 12 in human skin fibroblasts. UVB induced mostly C to T and G to A transitions and C to A and G to T transversions. The base-pair change with the highest relative abundance was C to T in the middle position of codon 11 followed by (in diminishing relative abundance) C to A in the middle position of codon 11, G to A and G to T in the middle position of codon 12. The C to T and G to A transitions are compatible with pyrimidine photodimers as pre-mutagenic lesions, whereas the C to A and G to T transversions could be generated due to the formation of 8-hydroxyguanine, which is the major oxidation product of guanine. The relative abundance of mutations induced by UVB in Ha-ras codons 11 and 12 does not correlate with mutations observed in the DNA from non-melanoma skin cancer, where the G to T transversion in the middle position of codon 12 is selected.

Ultraviolet B light-induced mutagenesis of p53 hotspot codons 248 and 249 in human skin fibroblasts

Mutations in the p53 tumor suppressor gene are detected in approximately half of non-melanoma skin cancers. The type of base-pair changes observed strongly suggests solar radiation as the causative mutagen. Mutations are distributed nonrandomly and form moderate hotspots. We studied the capacity of ultraviolet B light (UVB, 280-320 nm) to induce base-pair changes into the p53 exon 7 sequence extending from nt 14067 to 14075 in human skin fibroblasts. This sequence contains hotspot codon 248. UVB induced mostly C-->A and G-->T transversions. The base-pair change with the highest relative abundance was C-->A in the first position of codon 250 (CCC-->ACC), followed by (in diminishing relative abundance) G-->T in the third position of codon 249 (AGG-->AGT), C-->A in the first position of codon 248 (CGG-->AGG), and C-->A in the third position of codon 247 (AAC-->AAA). The C-->T transition in the third position of codon 247 (AAC-->AAT) occurred with moderate efficiency. These base-pair changes are compatible with pyrimidine photodimers as premutagenic lesions, but they could also form opposite 8-hydroxyguanine, which is the major oxidation product of guanine. No evidence was obtained for the presence of tandem double CC-->TT transitions in the untranscribed strand at codons 247/248 and 250. The relative abundance of mutations induced by UVB in the p53 sequence extending from codon 247 to 250 in human fibroblasts does not correlate with mutations observed in the DNA from non-melanoma skin cancer. This lack of correlation suggests that the mutability of this p53 sequence at the DNA level plays only a minor role in the pathogenesis of non-melanoma skin cancer in humans.

Mutations in a shuttle vector exposed to activated mitomycin C

The cytotoxicity of the potent antibiotic and antitumor agent mitomycin C (MMC) is due to its irreversible binding to DNA. Alkylating species generated by bioreductive activation of MMC are known to cause monoadducts and cross-links in DNA by specifically binding to guanine residues. To gain insight into how these lesions lead to base- and sequence-specific mutations, shuttle vector pSP189 was treated with MMC chemically reduced by treatment with sodium borohydride, replicated in human Ad293 cells, rescued in bacteria, and analyzed for mutations in the supF tRNA gene sequence. The MMC-induced mutations were predominantly base substitutions. Eighty-four percent of the base substitutions were transversions, with G:C-->T:A the major transversion. Single base deletions were the other major mutational event, and 77% of these were G:C deletions. Base positions 115, 123, and 163 were mutational hot spots based on the frequency of independent mutations. Identification of a single MMC adduct (presumed to be a modified G on the basis of its Rf value) and clustering of MMC-induced mutations at three GC-rich areas (nt 100-123, 152-163, and 168-176) suggested that the mutational spectrum we found was due to binding of MMC to guanine on either strand of the plasmid DNA.

Aflatoxin B1 induces the transversion of G-->T in codon 249 of the p53 tumor suppressor gene in human hepatocytes

Approximately half of hepatocellular carcinoma (HCC) from regions in the world with high contamination of food with the mycotoxin aflatoxin B1 (AFB1) contain a mutation in codon 249 of the p53 tumor suppressor gene. The mutation almost exclusively consists of a G-->T transversion in the third position of this codon, resulting in the insertion of serine at position 249 in the mutant protein. To gain insight into the mechanism of formation of this striking mutational hot spot in hepatocarcinogenesis, we studied the mutagenesis of codons 247-250 of p53 by rat liver microsome-activated AFB1 in human HCC cells HepG2 by restriction fragment length polymorphism/polymerase chain reaction genotypic analysis. AFB1 preferentially induced the transversion of G-->T in the third position of codon 249. However, AFB1 also induced G-->T and C-->A transversions into adjacent codons, albeit at lower frequencies. Since the latter mutations are not observed in HCC it follows that both mutability on the DNA level and altered function of the mutant serine 249 p53 protein are responsible for the observed mutational hot spot in p53 in HCC from AFB1-contaminated areas. Our results are in agreement with an etiological role of AFB1 in hepatocarcinogenesis in regions of the world with AFB1-contaminated food.

Translesional synthesis on a DNA template containing a single stereoisomer of dG-(+)- or dG-(-)-anti-BPDE (7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene)

Oligodeoxynucleotides modified site-specifically with dG-(+)-trans- and dG-(+)-cis-anti-BPDE (7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene) or dG-(-)-trans- and dG-(-)-cis-anti-BPDE were used as templates in primer extension reactions catalyzed by the Klenow fragment of Escherichia coli DNA polymerase I. The primer could be extended past the dG-(-)-trans-BPDE adduct with small amounts of dAMP incorporated opposite the lesion. A small amount of base deletions was also observed while, with the dG-(-)-cis-BPDE adduct, one- and two-base deletions predominated. When templates containing dG-(+)-trans-BPDE were used, small amounts of products containing one-base deletions were observed; with dG-(+)-cis-BPDE, substitution of dAMP opposite the lesion was also detected. The frequency of nucleotide insertion for dAMP opposite dG-(-)-trans-BPDE and the frequency of extension from the primer terminus containing the dA:dG-(-)-trans-BPDE pair were much higher than those observed with the other, stereochemically different BPDE adducts. Kinetic studies were in agreement with the results of the primer extension study. When the base flanking the 5' side of dG-BPDE was changed from dC to dT, the frequency of one-base deletions increased. We conclude that the trans- or cis-addition product of dG-(-)-anti-BPDE has a higher miscoding potential than dG-(+)-anti-BPDE in our model system and that G-->T transversions and deletions predominate. These observations are consistent with the types of mutations observed in vivo.

Spontaneous cleavage of bleomycin-induced abasic sites in chromatin and their mutagenicity in mammalian shuttle vectors

The stability of oxidized abasic sites induced by bleomycin and neocarzinostatin was examined in chromatin reconstituted from a supercoiled plasmid and core histones. Most of the drug-induced abasic sites were found to undergo spontaneous cleavage in chromatin, probably by reaction with histone amine groups. However, there was considerable heterogeneity in the rate of spontaneous cleavage, with some sites being cleaved almost immediately and some remaining intact even after 7 h. Bleomycin-induced abasic sites with closely opposed strand breaks were more unstable than lone abasic sites. Neocarzinostatin-induced abasic sites, which have a different chemical structure, were cleaved somewhat more slowly than those induced by bleomycin. To assess the mutagenic potential of bleomycin-induced abasic sites, bleomycin-treated shuttle vectors were transfected into mammalian cells, and mutations in progeny plasmids were sequenced. Bleomycin treatment resulted primarily in deletions of various sizes in the shuttle vectors, including a number of one-base deletions occurring at potential bleomycin damage sites. However, under certain conditions, substitutions occurring at expected sites of bleomycin attack were also observed. The results suggest that bleomycin-induced abasic sites have only a slight potential to produce base substitutions in mammalian cells and that a substantial fraction of the double-strand breaks induced by bleomycin and most of the double-strand breaks induced by neocarzinostatin are the result of spontaneous cleavage of abasic sites with closely opposed strand breaks. Inaccurate repair of these double-strand breaks may account for the large deletions, and perhaps the one-base deletions, induced by bleomycin.

Quantitative comparison of initiation and mutation phenotypes in hepatocytes of the analbuminemic rat

The potential relationship between mutagenesis and carcinogenesis has been examined in the Nagase analbuminemic rat treated with a single dose of benzo[a]pyrene, an incomplete liver carcinogen. The apparent mutation rate at the albumin locus was calculated by determining the number of hepatocytes expressing a cross-reactive product of albumin in analbuminemic rats treated with benzo[a]pyrene. The rate of initiation, the first stage in carcinogenesis, was determined by assessing the number of hepatocytes expressing the placental isozyme of glutathione S-transferase (PGST) after administration of benzo[a]pyrene. Since the expression of PGST may represent hepatocellular changes independent of initiation, promotion with phenobarbital was employed to clonally expand those putatively initiated hepatocytes expressing PGST. With immunohistochemical measures to assess changes in albumin expression, a threefold increase in the number of hepatocytes expressing albumin was detected after administration of benzo[a]pyrene in Nagase analbuminemic rats. A more than five-fold increase in altered hepatic foci (AHF) exhibiting increased PGST expression was observed in animals given benzo[a]pyrene treatment followed by phenobarbital, compared with those given benzo[a]pyrene alone. The number of albumin-expressing single hepatocytes detected was of the same order of magnitude as the number of individual hepatocytes and AHF expressing PGST, suggesting that similar events may be involved in their formation. Since 3 x 10(6) single hepatocytes expressing albumin were found in the analbuminemic rat liver after a single administration of benzo[a]pyrene, while less than 2 x 10(4) AHF expressing PGST were observed, formation of individual hepatocytes expressing albumin was a far more frequent event than clonal expansion of initiated hepatocytes in the Nagase analbuminemic rat. However, the number of loci of PGST expression including AHF and single hepatocytes is comparable to that of single hepatocytes expressing albumin.

Mutagenic specificity of the (+)anti-diol epoxide of dibenz[a,j]anthracene in the supF gene of an Escherichia coli plasmid

This study was designed to examine the mutagenic specificity of (+)anti-dibenz[a,j]anthracene 3,4-diol-1,2-epoxide ((+)anti-DB[a,j]A-DE) in SOS-induced repair-proficient Escherichia coli ES87 (delta pro-lac, strA)/F' (pro+, lac1Q, lac1am26, lacZ delta M15). The plasmid pUB3, which contains the mutation target gene, supF, was modified with (+)anti-DB[a,j]A-DE in vitro (two to five adducts/plasmid) and then transformed into bacteria by electroporation. The spontaneous mutation frequency for unmodified pUB3 in uninduced cells was about 2 x 10(-6) and for SOS-induced cells, about 8 x 10(-6). The spontaneous supF- mutations were primarily insertions, deletions, and frameshifts. The mutation frequency for (+)anti-DB[a,j]A-DE-modified pUB3 was about 8 x 10(-6) and about 32 x 10(-6) for uninduced cells and SOS-induced cells, respectively. (+)anti-DB[a,j]A-DE induced primarily point mutations in supF in SOS-induced cells. GC-->AT transitions were the major mutations observed in SOS-induced cells (37%). GC-->TA (21%) and GC-->CG (8.6%) transversion mutations were also observed, whereas mutations at AT base pairs were rare (1.9%). Furthermore, a large number of tandem GC/GC-->AT/AT transition mutations were also observed (about 15% of all mutations in SOS-induced cells). Taken together, single and tandem GC-->AT mutations accounted for slightly over half (about 51%) of the mutations observed in SOS-induced cells. These results demonstrated that (+)anti-DB[a,j]A-DE was mutagenic in repair-proficient E. coli; however, unlike other polycyclic aromatic hydrocarbons that induce primarily transversion mutations, (+)anti-DB[a,j]A-DE caused mostly GC-->AT transitions.

Determination of point mutational spectra of benzo[a]pyrene-diol epoxide in human cells

The primary goal of our research consists of developing means sufficiently sensitive to allow assessment of human exposure to environmental carcinogens. We describe here a new approach for analyzing point mutational spectra and a test for its validity and precision using cultured human cells exposed to high doses of environmental carcinogens. The approach in its present form includes a) treatment of independent large cultures of human cells with a carcinogen, b) selection of mutant cells en masse by 6-thioguanine resistance, c) amplification of sequences of interest directly from 6TGR cells using high-fidelity polymerase chain reaction, and d) separation of mutant sequences from nonmutant sequences using denaturing gradient gel electrophoresis. We report use of this protocol to observe induced mutational spectra in exon 3 of the hprt gene in cultured human cells by benzo[a]pyrene-diol epoxide (BPDE), an active form of the widely distributed environmental carcinogen benzo[a]pyrene. BPDE induced predominantly G to T transversions within this target sequence. The variation of the frequency of the mutations among independent cultures is consistent with the interpretation that each of them corresponds to a hotspot.

Mutational spectrometry: a general approach for hot-spot point mutations in selectable genes

To observe point mutational spectra with a high degree of precision, independent large cultures of human lymphoblastoid cells were treated with a mutagen, benzo[a]pyrene diol epoxide, and mutants at the HPRT gene were selected en masse by 6-thioguanine resistance. An average of 1.6 x 10(4) 6-thioguanine-resistant mutants were created per experiment and the kinds, positions, and numbers of the most frequent mutations were examined in exon 3 of the HPRT gene by using a high-fidelity polymerase chain reaction and denaturing gradient gel electrophoresis. Sixteen exon 3-specific mutations were found to be predominantly G----T transversions and corresponded to an average of 3500 induced mutants per experiment. Of these mutations, 6 occurred within a run of 6 guanines and 5 occurred in the sequence 5'-GAAGAG-3'. The variation among independent experiments is consistent with the numerical expectation that all 16 mutations fulfill reasonable statistical criteria for mutational hot spots. The agreement with data from various systems using clone-by-clone analysis shows that the protocol reported herein can be a useful tool to study hot-spot point mutational spectra for DNA sequences for which phenotypic selection systems exist.

Kinds of mutations induced by aflatoxin B1 in a shuttle vector replicating in human cells transiently expressing cytochrome P4501A2 cDNA

Transient expression of rat liver cytochrome P450lA2 cDNA was combined with the use of a shuttle vector as a mutational target to determine the frequency and types of mutation caused by the conversion of aflatoxin B1 into genotoxic metabolites within human cells. Ad293 cells were first transfected with p91-lA2, a rat liver P450lA2 cDNA expression vector, or with p91-lA2(i) (a control vector that has the P450 cDNA in the inverted orientation) and incubated for 24 h to permit P450lA2 accumulation. Cells were then transfected with the pS189 shuttle-vector plasmid, which carries the Escherichia coli supF gene as a mutational target, and incubated for a further 24 h in the presence of aflatoxin B1 to permit promutagen activation and pS189 replication. In shuttle vectors replicated in p91-lA2-transfected cells, the supF point-mutation frequency increased with increasing concentration of aflatoxin B1. This frequency was nine to 23 times greater than the background point-mutation frequency obtained with aflatoxin B1-treated control (p91-lA2(i)-transfected) cells. The large majority of the aflatoxin B1-induced supF point mutations were base substitutions, mostly G:C----T:A transversions. This mutagenesis system permits the molecular analysis of mutations induced by specific P450/promutagen pairs in a shuttle vector replicating in human cells and will permit the investigation of host cell mechanisms involved in the generation of these mutations.

Dose-dependent differences in the profile of mutations induced by an ultimate carcinogen from benzo[a]pyrene

Mutations in the coding region of the hypoxanthine (guanine) phosphoribosyltransferase (HPRT) gene of Chinese hamster V-79 cells were examined after exposure of the cells to a high cytotoxic dose (0.48 microM; 35% survival) and a low noncytotoxic dose (0.04 microM; 100% survival) of the ultimate carcinogen (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+)-BPDE]. Independent 8-azaguanine-resistant colonies were isolated and cDNAs were prepared by reverse transcription. The coding region of the cDNA of the HPRT gene was amplified by the polymerase chain reaction and sequenced. An examination of the DNA base sequence changes induced by different doses of (+)-BPDE demonstrated that the high dose of (+)-BPDE caused base substitution mutations almost exclusively at G.C base pairs whereas the low dose of (+)-BPDE caused mutations at both G.C and A.T base pairs. Thus, use of a low dose of (+)-BPDE allowed the detection of mutations (at A.T base pairs) that were not readily observed with a high dose of (+)-BPDE. The data also suggest that the low dose of (+)-BPDE may have caused a different profile of base substitutions at G.C base pairs and exon deletions than the high dose. The results indicate dose-dependent differences in the profile of mutations for an ultimate carcinogen.

Replication of acetylaminofluorene-adducted plasmids in human cells: spectrum of base substitutions and evidence of excision repair

In rats fed the liver carcinogen 2-acetylaminofluorene (AAF), the two most abundant types of DNA adduct are N-(deoxyguanosin-8-yl)-2-acetylaminofluorene and its deacetylated derivative. When plasmids carrying AAF adducts replicate in bacteria, the predominant mutations are frameshifts, whereas with deacetylated (AF) adducts, they are mainly base substitutions, just as we found when plasmids carrying AF adducts replicated in human cells. We have investigated the frequency and spectrum of mutations induced when a shuttle vector carrying AAF adducts (85% bound to the C8 position of guanine, 15% to the N2 position) replicated in human cells. The frequency induced per initial AAF adduct was higher than with AF adducts, but the kinds of mutations were similar--i.e., 85% base substitutions, principally G.C----T.A transversions. There was good correlation between the "hot spots" for mutations and hot spots for AAF adduct formation, suggesting that mutational hot spots reflect preferential binding of the carcinogen to DNA. 32P-postlabeling analysis of the adducts before and after the DNA was transfected into the human cells showed that there was no deacetylation of AAF adducts and that 85% of both types of adducts were removed within 3.5 hr, most probably by excision repair.

Xeroderma pigmentosum variant cells are less likely than normal cells to incorporate dAMP opposite photoproducts during replication of UV-irradiated plasmids

Xeroderma pigmentosum (XP) variant patients show the clinical characteristics of the disease, with increased frequencies of skin cancer, but their cells have a normal, or nearly normal, rate of nucleotide excision repair of UV-induced DNA damage and are only slightly more sensitive than normal cells to the cytotoxic effect of UV radiation. However, they are significantly more sensitive to its mutagenic effect. To examine the mechanisms responsible for this hypermutability, we transfected an XP variant cell line with a UV-irradiated (at 254 nm) shuttle vector carrying the supF gene as a target for mutations, allowed replication of the plasmid, determined the frequency and spectrum of mutations induced, and compared the results with those obtained previously when irradiated plasmids carrying the same target gene replicated in a normal cell line [Bredberg, A., Kraemer, K. H. & Seidman, M. M. (1986) Proc. Natl. Acad. Sci. USA 83, 8273-8277]. The frequency of mutants increased linearly with dose, but with a slope 5 times steeper than that seen with normal cells. Sequence analysis of the supF gene showed that 52 of 53 independent mutants generated in the XP variant cells contained base substitutions, with 62 of 64 of the substitutions involving a dipyrimidine. Twenty-eight percent of the mutations involved A.T base pairs, with the majority found at position 136, the middle of a run of three A.T base pairs. (In the normal cells, this value was only 11%.) If the rate of excision of lesions from supF in the two cell lines is equal, our data suggest that XP variant cells are less likely than normal cells to incorporate dAMP opposite bases involved in photo-products. If such incorporation also occurs during replication of chromosomal DNA, this could account for the hypermutability of XP variant cells with UV irradiation.

Dependence of the mutation spectrum in a shuttle plasmid replicated in human lymphoblasts on dose of gamma radiation

The frequencies and types of mutations induced in the target gene, supF-tRNA, of the shuttle vector pZ189 were analysed following the replication of the gamma-irradiated plasmid in the human lymphoblastoid cell line, GM606. The mutation frequency measured in progeny of unirradiated pZ189 was 1.02 x 10(-4), increasing to 17.5 x 10(-4) at 1000 cGy, and to 63.4 x 10(-4) at 5000 cGy, approximately 17- and 62-fold over background levels, respectively. Simultaneously, the number of plasmids capable of replicating in Escherichia coli decreased with increasing radiation dose to 4% of the control value at 5000 cGy. Electrophoresis of the irradiated DNA showed a correlation between increases in mutation frequency and decreases in plasmid survival, and the formation of open-circular and linear DNA. The majority of the spontaneous (69.8%) and induced mutations (85.7%) at 1000 and 79.4% at 5000 cGy) were base substitutions and were generally of similar types among all groups. However, changes at 2500 (12.7%) and 5000 cGy (13.2%) involving A:T base pairs were greater than those in unirradiated controls (3.4%) or those at 1000 cGy (2.0%). This increase in A:T base pair mutations could be a result of reduced repair fidelity when the DNA is extensively damaged by high doses of ionizing radiation.